The two fastest-growing church bodies in the United States and Canada, according to a newly published report, are ones whose beliefs are known to conflict with traditional Christian teaching.

Jehovah's Witnesses and the Church of Jesus Christ of Latter-day Saints, regarded by many Christians as cults, reported the largest membership increases in a year, according to the National Council of Churches' 2008 Yearbook of American & Canadian Churches.

Although Jehovah's Witnesses currently rank 25th in size with over 1.06 million members, they reported a 2.25 per cent increase in membership since the publication of the 2007 Yearbook. The Church of Jesus Christ of Latter-day Saints – also known as the Mormon church – grew 1.56 per cent and is listed by the NCC as the fourth largest 'church'.

Notably, however, both Jehovah's Witnesses and the Mormon church are not accepted within many Christian circles as part of the larger Body of Christ over a number of controversial beliefs that the two religions hold.

The identification of Jehovah's Witnesses as Christian is debated largely due to their rejection of the Trinity, which most Christians regard as a fundamental doctrine. Latter-day Saints, meanwhile, are often criticised for their belief in "divine" books of scripture, aside from the Bible, including the Book of Mormon, the Doctrine and Covenants, and the Pearl of Great Price.

Mormonism was formally listed under "cults and sects" by the Southern Baptist Convention – the largest Protestant denomination in the nation – but was more recently categorised among "newly developed religions" on the North American Mission Board apologetics page.

Other bodies in the newly published top 25 largest churches list that reported membership increases include the Catholic Church with a 0.87 per cent increase; the Southern Baptist Convention with a 0.22 per cent increase; the African Methodist Episcopal Zion Church with a 0.21 per cent rise; and the Assemblies of God with a 0.19 per cent growth.

The greatest losses in membership were reported by the Episcopal Church, which dropped 4.15 per cent in members, and the Presbyterian Church (USA), which decreased by 2.36 per cent. Both denominations are currently wracked by theological differences and the issue of homosexuality.

American Baptist Churches in the USA and the Evangelical Lutheran Church in America also experienced large losses in membership, dropping 1.82 per cent and 1.58 per cent, respectively.

"Some will wish to argue that the slowing growth rate is evidence of an increasing secularisation of American postmodern society," said the Rev Dr Eileen W Lindner, editor of the Yearbook. "While such an explanation will satisfy some, caution in drawing such a conclusion is warranted."

Lindner also observed that churches are feeling the impact of the lifestyles of Millenials – people in their 20s and 30s – who attend church but resist becoming members.

The United Methodist Church saw a 0.99 per cent decrease but the mainline group remains the third largest church body with nearly 8 million members.

Only three of the top 10 largest churches are mainline Protestant churches; three of the top 25 are Pentecostal churches; and six of the top 15 are historic African American churches.